(The opinions expressed here are those of the author, a columnist for Reuters)
* Graphic - tmsnrt.rs/2qjlJHL
By Andy Home
LONDON, May 15 (Reuters) - If there were any doubt that the zinc supply chain is tightening, it should be dispelled by this year's benchmark smelter treatment charge.
The treatment charge is the fee paid by a miner to a smelter for converting mined concentrates into refined metal and it is probably the best indicator of raw material availability; high during times of surplus and low during times of scarcity.
This year's headline fee of $172 per tonne is the lowest in a decade, a firm swing of the negotiating pendulum in favour of miners and a tangible sign that the much-anticipated concentrates crunch has arrived.
Indeed, miners have used the squeeze on availability to make what might turn out to be a historic change in how these annual benchmark contracts are structured.
Zinc bulls have been waiting a long time for this supply squeeze and they may have to wait a bit longer before it moves from raw materials to refined metal parts of the chain.
But at a mined concentrates level it has very surely arrived.
Graphic on annual zinc treatment charges:
This year's headline treatment charge of $172 per tonne was confirmed by Belgium's Nyrstar, a zinc miner itself but a much bigger converter of concentrates into refined metal.
It represented a 15 percent decline from last year's benchmark of $203 per tonne and was the lowest outcome since 2006.
The comparison is worth noting because that was the year London Metal Exchange (LME) zinc hit its highest ever level at $4,580 per tonne.
As well as seeing their revenues reduced this year, smelters have also lost any price participation.
Price participation disappeared from copper concentrate contracts several years ago but it has persisted in the zinc market in the weird and wonderful form of "escalators" and "de-escalators".
These determine how much the treatment charge can change depending on how far the zinc price deviates from a preset "basis" price in the contract.
In 2016, for example, the "basis" price was set at $2,000 with escalators allowing for price participation up to $3,750 and de-escalators down to $1,500.
This year, however, both escalator and de-escalator have been set at zero, a partial victory for those miners seeking the complete elimination of price participation by smelters.
Escalators and de-escalators remain in the benchmark contract, in theory allowing for a resurrection at a future date.
Whether price participation actually returns remains to be seen.
When BHP Billiton dropped price participation from its copper contracts in 2007, it was at the time presented as a temporary market-driven phenomenon.
But within a couple of years everyone else had done the same and price participation was quietly consigned to the copper history books.
Smelters can probably afford to be sanguine about the zero price participation this year.
As Hilmar Rode, chief executive of Nyrstar, told analysts on the company's first-quarter conference call: "If you go back and you monitor say the last 10 years, you'll see sometimes that works in favour of the mine sometimes in favour of the smelters but over a longer period of time ... it's pretty much a net zero."
More important to a company like Nyrstar is the retention of another strange-sounding component of the annual zinc treatment charge benchmark, namely "free metal".
What this means is that zinc smelters pay for only 85 percent of the metal contained in the concentrate.
This curiosity dates back to a long-lost time when most of the world's zinc smelters were pyrometallurgical and typically could only extract that amount of metal from the concentrate.
Times and technology have changed a lot since then and a company such as Nyrstar can now typically recover around 96-97 percent of the metal, meaning it gets 11-12 percent "free".
Miners would no doubt love to eat into, if not eliminate altogether, this "free metal" allowance but the consensus seems to be that this would be a step too far right now.
The sharp drop in the benchmark treatment charge was widely expected.
This zinc supply crunch has been a long time coming and there have been plenty of false starts for over-eager bulls in recent years.
But, to quote Jonathan Leng, principle zinc analyst at Wood Mackenzie, "the record 6.3 percent fall in global mine supply in 2016 transformed the concentrate market."
Concentrate stocks fell to "minimum working levels" in September of last year and smelters, particularly those in China, are having to cut production.
Nor does Woodmac see much change in the zinc concentrates market any time soon. Its view is that it will remain tight for the next couple of years with treatment charges likely to remain at correspondingly low levels over that period.
What does this mean for the refined zinc price?
So far bulls have been frustrated that mine supply crunch hasn't translated into refined metal crunch.
China's imports of refined zinc remain subdued, while metal is still occasionally appearing on LME warrant at New Orleans, albeit not in the sort of volumes as seen in the past.
Woodmac's Leng, however, believes it's only a matter of time. He expects to see acute tightness later this year with stocks "projected to fall to historically low levels and remain so until 2020."
That will translate into a price high next year "comparable to the 2006 price peak".
Not everyone, it's fair to say, is quite so bullish . There are still a good number of known unknowns at work, not least the state of mine supply in China itself.
But this year's benchmark concentrates terms make it hard to argue against the starting proposition of bulls such as Wood Mackenzie.
The concentrates market is as tight as it's been since 2006.
Whether that means a return to the historical peak in zinc prices in that year remains to be seen.
Editing by David Evans